Abstract
Previous cochlear implant (CI) research has shown that at a pulse train with a long pulse phase duration (PPD) requires less current but greater charge to obtain the same loudness as a pulse train with a short PPD. This might result in different excitation patterns between long and short PPDs. At equal loudness, long PPDs might produce greater masking due to greater charge. However, because they require less current, long PPDs may produce a smaller spatial spread of excitation (SOE) compared to short PPDs by evoking a greater neural firing probability within the relatively small current field. To investigate the effects of PPD on excitation patterns, overall masking and SOE were compared for equally loud stimuli with short or long PPD in 10 adult CI ears. Forward masking patterns were measured at relatively soft, medium, and loud presentation levels. Threshold shifts were calculated in terms of percent dynamic range (DR) of the probe. The area under the curve (AUC) of the masking functions was significantly larger for the long PPD than for the short PPD masker. The difference in AUC was proportional to the difference in charge between the short and long PPD maskers. To estimate SOE, the masking patterns were first normalized to the peak masking, and then AUC was calculated. SOE was significantly larger for the short PPD than for the long PPD masker. Thus, at equal loudness, long PPDs produced greater overall masking (possibly due to greater charge) but less SOE (possibly due to less current spread) than did short PPDs. The effect of the interaction between masking and SOE by long PPD stimulation remains to be tested.
Highlights
Most modern cochlear implant (CI) devices encode intensity by adjusting the pulse amplitude (PA) with a fixed pulse phase duration (PPD)
We examined the differences in forward masking and spatial spread of neural excitation (SOE) produced by short and long PPDs
Results showed that spread of excitation (SOE) was largely unchanged across presentation levels for both short and long PPD maskers
Summary
Most modern cochlear implant (CI) devices encode intensity by adjusting the pulse amplitude (PA) with a fixed pulse phase duration (PPD). In most CI systems, the PPD is typically short ( 25 μs/phase), allowing for high stimulation rates and rapid loudness growth with increasing PA. Some CI devices currently encode intensity by varying PPD with a fixed PA. PPD may be increased beyond default values to reduce facial nerve stimulation, to accommodate patients with auditory neuropathy spectrum disorders, or to remain within voltage compliance [1]. Perceptual differences between long and short PPDs are not well understood. We examined the differences in forward masking and spatial spread of neural excitation (SOE) produced by short and long PPDs. The results may provide insights into optimizing PPD for individual patients in clinical fitting
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